SLEEP IS A REMARKABLE STATE. It consumes fully a third of our lives—approximately 25 years in the average lifetime—yet we know little about what happens in the brain during this daily excursion. Perhaps even more surprising, the exact functions of sleep and of dreaming, one of the more noteworthy components of sleep, are still unknown.
Although the psychological content of dreams has been a rich subject of speculation from Plato and Aristotle to Sigmund Freud, we still do not understand whether dreams carry deep personal meaning, as Freud hypothesized, or represent the brain “throwing out its trash,” the bits and pieces of daily experience that are not worth retaining, as Francis Crick speculated. One function of sleep may be to allow synaptic remodeling and consolidation of memory traces reflecting the day’s experiences, but the role of dreaming in this process remains a subject of intense debate.
When studying sleep and wakefulness, researchers typically use a polysomnogram, which consists of three physiological measures: brain activity measured by an electroencephalogram (EEG) (see Figure 58–1), eye movements recorded by an electro-oculogram (EOG), and muscle tone measured by an electromyogram (EMG) (Figure 44–1B). In clinical polysomnograms, respiration is also measured, as breathing during sleep is disrupted in many patients with sleep disorders.
Electrophysiological patterns of wakefulness and sleep.
A. A hypnogram or graph showing the progression of sleep stages over a typical night in a healthy young person. Periods of rapid eye movement (REM) sleep alternate with non-REM sleep about every 90 minutes. An individual typically progresses from the awake state into light non-REM sleep (N1) then progressively deeper non-REM sleep (N2, N3), then back to lighter non-REM sleep before the first period of REM sleep occurs (light blue bars). As the night progresses, the individual spends less time in the deepest stage of non-REM sleep, and the duration of REM sleep periods increases.
B. The records show the components of the polysomnogram used to distinguish sleep stages. The electro-oculogram (EOG) records eye movements from electrodes on either side of the eyes. The electroencephalogram (EEG) records cortical field potentials from the scalp; the electromyogram (EMG) records muscle fiber firing through the skin. During the awake state, the EOG shows voluntary eye movements, the EEG shows fast low-amplitude activity, and the EMG shows variable muscle tone. Stage N1 sleep is characterized by a slight slowing of EEG frequencies and slow roving eye movements, with less EMG activity; stage N2 is characterized by bursts of 12- to 14-Hz activity called sleep spindles and high-voltage slow waves called K-complexes; stage N3 is dominated by high-voltage slow waves. During REM sleep, the EEG is similar to that of the awake state. Rapid eye movements can be seen on the EOG, but the EMG is so silent that contamination by tiny electrocardiogram signals can sometimes be ...